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Migration Guide for NX

Abstract

This document describes the differences between the Next Scripting Language Framework and XOTcl 1. In particular, it presents a migration guide from XOTcl 1 to NX, and presents potential incompatibilities beween XOTcl 1 and XOTcl 2.

The Next Scripting Language (NX) is a successor of XOTcl 1 and is based on 10 years of experience with XOTcl in projects containing several hundert thousand lines of code. While XOTcl was the first language designed to provide language support for design patterns, the focus of the Next Scripting Framework and NX are on combining this with Language Oriented Programming. In many respects, NX was designed to ease the learning of the language by novices (by using a more mainstream terminology, higher orthogonality of the methods, less predefined methods), to improve maintainability (remove sources of common errors) and to encourage developer to write better structured programs (to provide interfaces) especially for large projects, where many developers are involved.

The Next Scripting Language is based on the Next Scripting Framework which was developed based on the notion of language oriented programming. The Next Scripting Frameworks provides C-level support for defining and hosting multiple object systems in a single Tcl interpreter. The whole definition of NX is fully scripted (e.g. defined in nx.tcl). The Next Scripting Framework is shipped with three language definitions, containing NX and XOTcl 2. Most of the existing XOTcl 1 programs can be used without modification in the Next Scripting Framework by using XOTcl 2. The Next Scripting Framework requires Tcl 8.5 or newer.

Although NX is fully scripted (as well as XOTcl 2), our benchmarks show that scripts based on NX are often 2 or 4 times faster than the counterparts in XOTcl 1. But speed was not the primary focus on the Next Scripting Environment: The goal was primarily to find ways to repackage the power of XOTcl in an easy to learn environment, highly orthogonal environment, which is better suited for large projects, trying to reduce maintenance costs.

We expect that many user will find it attractive to upgrade from XOTcl 1 to XOTcl 2, and some other users will upgrade to NX. This document focuses mainly on the differences between XOTcl 1 and NX, but addresses as well potential incompatibilities between XOTcl 1 and XOTcl 2. For an introduction to NX, please consult the NX tutorial.

1. Differences Between XOTcl and NX

The Next Scripting Framework supports Language Oriented Programming by providing means to define potentially multiple object systems with different naming and functionality in a single interpreter. This makes the Next Scripting Framework a powerful instrument for defining multiple languages such as e.g. domain specific languages. This focus differs from XOTcl 1.

Technically, the language framework approach means that the languages implemented by the Next Scripting Framework (most prominently XOTcl 2 and NX) are typically fully scripted and can be loaded via the usual Tcl package require mechanism.

Some of the new features below are provided by the Next Scripting Framework, some are implemented via the script files for XOTcl 2 and NX.

1.1. Features of NX

In general, the Next Scripting Language (NX) differs from XOTcl in the following respects:

  1. Stronger Encapsulation: The Next Scripting Language favors a stronger form of encapsulation than XOTcl. Calling the own methods or accessing the own instance variables is typographically easier and computationally faster than these operations on other objects. This behavior is achieved via resolvers, which make some methods necessary in XOTcl 1 obsolete in NX (especially for importing instance variables). The encapsulation of NX is stronger than in XOTcl but still weak compared to languages like C++; a developer can still access other objects' variables via some idioms, but NX makes accesses to other objects variables explicit. The requiredness to make these accesses explicit should encourage developer to implement well defined interfaces to provide access to instance variables.

  2. Additional Forms of Method Definition and Reuse: The Next Scripting Language provides much more orthogonal means to define, reuse and introspect scripted and C-implemented methods.

    1. It is possible to use NX alias to register methods under arbitrary names for arbitrary objects or classes.

    2. NX provides means for method protection (method modifiers public, protected and private). Therefore developers have to define explicitly public interfaces in order to use methods from other objects.

    3. One can invoke in NX fully qualified methods to invoke methods outside the precedence path.

    4. One can define in NX hierachical method names (similar to commands and subcommands, called method ensembles) in a convenient way to provide extensible, hierarchical naming of methods.

    5. One can use in NX the same interface to query (introspect) C-implemented and scripted methods/commands.

  3. Orthogonal Parameterization: The Next Scripting Language provides an orthogonal framework for parametrization of methods and objects.

    1. In NX, the same argument parser is used for

      • Scripted Methods

      • C-implemented methods and Tcl commands

      • Object Parametrization

    2. While XOTcl 1 provided only value-checkers for non-positional arguments for methods, the Next Scripting Framework provides the same value checkers for positional and non-positional arguments of methods, as well as for positional and non-positional object parameters (-parameter in XOTcl 1).

    3. While XOTcl 1 supported only non-positional arguments at the begin of the argument list, these can be used now at arbitrary positions.

  4. Value Checking:

    1. The Next Scripting Language supports checking of the input parmeters and the return values of scripted and C-implemented methods and commands.

    2. NX provides a set of predefined checkers (like e.g. integer, boolean, object, …) which can be extended by the applications.

    3. Value Checking can be used for single and multi-valued parameters. One can e.g. define a list of integers with at least one entry by the parameter specification integer,1..n.

    4. Value Checking can be turned on/off globally or on the method/command level.

  5. Scripted Init Blocks: The Next Scripting Language provides scripted init blocks for objects and classes (replacement for the dangerous dash "-" mechanism in XOTcl that allows to set variables and invoke methods upon object creation).

  6. More Conventional Naming for Predefined Methods: The naming of the methods in the Next Scripting Language is much more in line with the mainstream naming conventions in OO languages. While for example XOTcl uses proc and instproc for object specific and inheritable methods, NX uses simply method.

  7. Profiling Support: The Next Scripting Language provides now two forms of profiling

    • Profiling via a DTrace provider (examples are e.g. in the dtrace subdirectory of the source tree)

    • Significantly improved built-in profiling (results can be processed in Tcl).

  8. Significantly Improved Test Suite: The regression test suite of Next Scripting Scripting framework contain now more than 5.000 tests, and order of magnitude more than in XOTcl 1.6

  9. Much Smaller Interface: The Next Scripting Language has a much smaller interface (i.e. provides less predefined methods) than XOTcl (see Table 1), although the expressability was increased in NX.

Table 1. Comparison of the Number of Predefined Methods in NX and XOTcl
NX XOTcl

Total

44

124

Methods for Objects

20

51

Methods for Classes

3

24

Info-methods for Objects

15

25

Info-methods for Classes

6

24

This comparison list compares mostly XOTcl 1 with NX, some features are also available in XOTcl 2 (2a, 2c 2d, 3, 4).

1.2. NX and XOTcl Scripts

Below is a small, introductory example showing an implementation of a class Stack in NX and XOTcl. The purpose of this first example is just a quick overview. We will go into much more detailed comparison in the next sections.

NX supports a block syntax, where the methods are defined during the creation of the class. The XOTcl syntax is slightly more redundant, since every definition of a method is a single toplevel command starting with the class name (also NX supports the style used in XOTcl). In NX, all methods are per default protected (XOTcl does not support protection). In NX methods are defined in the definition of the class via :method or :public method. In XOTcl methods are defined via the instproc method.

Another difference is the notation to refer to instance variables. In NX, instance variable are named with a single colon in the front. In XOTcl, instance variables are imported using instvar.

Stack example in NX Stack example in XOTcl
Class create Stack {

   #
   # Stack of Things
   #

   :variable things ""

   :public method push {thing} {
      set :things [linsert ${:things} 0 $thing]
      return $thing
   }

   :public method pop {} {
      set top [lindex ${:things} 0]
      set :things [lrange ${:things} 1 end]
      return $top
   }
}
#
# Stack of Things
#

Class Stack Stack instproc init {} { my instvar things set things "" } Stack instproc push {thing} { my instvar things set things [linsert $things 0 $thing] return $thing } Stack instproc pop {} { my instvar things set top [lindex $things 0] set things [lrange $things 1 end] }

1.3. Using XOTcl 2.0 and the Next Scripting Language in a Single Interpreter

In general, the Next Scripting Framework supports multiple object systems concurrently. Effectively, every object system has different base classes for creating objects and classes. Therefore, these object systems can have different different interfaces and names of built-in methods. Currently, the Next Scripting Framework is packaged with three object systems:

  • NX

  • XOTcl 2.0

  • TclCool

XOTcl 2 is highly compatible with XOTcl 1, the language NX is described below in more details, the language TclCool was introduced in Tip#279 and serves primarily an example of a small OO language.

A single Tcl interpreter can host multiple Next Scripting Object Systems at the same time. This fact makes migration from XOTcl to NX easier. The following example script shows to use XOTcl and NX in a single script:

Using Multiple Object Systems in a single Script
namespace eval mypackage {

  package require XOTcl 2.0

  # Define a class using XOTcl
  xotcl::Class C1
  C1 instproc foo {} {puts "hello world"}

  package require nx

  # Define a class using NX
  nx::Class create C2 {
    :public method foo {} {puts "hello world"}
  }
}

One could certainly create object or classes from the different object systems via fully qualified names (e.g. using e.g. ::xotcl::Class or ::nx::Class), but for migration for systems without explicit namespaces switching between the object systems eases migration. "Switching" between XOTcl and NX effectively means the load some packages (if needed) and to import either the base classes (Object and Class) of XOTcl or NX into the current namespace.

2. XOTcl Idioms in the Next Scripting Language

The following sections are intended for reader familiar with XOTcl and show, how certain language Idioms of XOTcl can be expressed in NX. In some cases, multiple possible realizations are listed

2.1. Defining Objects and Classes

When creating objects or classes, one should use the method create explicitly. In XOTcl, a default unknown method handler was provided for classes, which create for every unknown method invocation an object/class with the name of the invoked method. This technique was convenient, but as well dangerous, since typos in method names lead easily to unexpected behavior. This default unknown method handler is not provided in NX (but can certainly be provided as a one-liner in NX by the application).

XOTcl Next Scripting Language
Class ClassName
Class create ClassName
Object ObjectName
Object create ObjectName

2.2. Defining Methods

In general, both XOTcl and NX support methods on the object level (per-object methods, i.e. methods only applicable to a single object) and on the class level (methods inherited to instances of the classes). While the naming in XOTcl tried to follow closely the Tcl tradition (using the term proc for functions/methods), NX uses the term method for defining scripted methods.

XOTcl uses the prefix inst to denote that methods are provided for instances, calling therefore scripted methods for instances instproc. This is certainly an unusual term. The approach with the name prefix has the disadvantage, that for every different kind of method, two names have to be provided (eg. proc and instproc, forward and instforward).

NX on the contrary uses the same term for defining inherited or object-specific methods. When the term (e.g. method) is used on a class, the method will be inherited (applicable to the instances of the class). When the term is used on an object, an object-specific method is defined. NX uses the method modifier class to define a class-specific method (method for the class object).

Furthermore, both XOTcl and NX distinguish between scripted methods (section 3.2.1) and C-defined methods (section 3.2.2). Section 3.2.3 introduces method protection, which is only supported by NX.

2.2.1. Scripted Methods Defined in the Init-block of a Class/Object or with Separate Calls

The following examples show the definition of a class and its methods in the init-block of a class (NX only), and the definition of methods via separate top level calls (XOTcl and NX).

XOTcl Next Scripting Language
# Define method 'foo' and class
# method 'bar' for a Class 'C' with separate
# toplevel commands

Class C C instproc foo args {...} C proc bar args {...}
# Define method and class method
# in the init-block of a class

Class create C { :method foo args {...} :class method bar args {...} }
# Define method and class method
# with separate commands

Class create C C method foo args {...} C class method bar args {...}
# Define object-specific method foo
# for an object 'o' with separate commands

Object o o set x 1 o proc foo args {...}
# Define class method and set
# instance variable in the init-block of
# an object

Object create o { set :x 1 :method foo args {...} }
# Define class method and set
# instance variable with separate
# commands

Object create o o eval {set :x 1} o method foo args {...}

2.2.2. Different Kinds of Methods

This section describes various kinds of methods. The different kinds of methods are defined via different method-defining methods, which are summarized in the following table for XOTcl and NX.

XOTcl Next Scripting Language
# Methods for defining methods:
#
#     proc
#     instproc
#     forward
#     instforward
#     parametercmd
#     instparametercmd
#
# All these methods return empty.
# Methods for defining methods:
#
#     method
#     forward
#     alias
#     property
#
# All these methods return method-handles.

In addition to scripted methods (previous section) XOTcl supports forwarder (called forward and instforward) and accessor functions to variables (called parametercmd and instparametercmd). The accessor functions are used normally internally when object-specific parameters are defined (see Section 3.4).

In NX forwarders are called forward. NX does not provide an own method to define variable accessors, but uses the Next Scripting Framework primitive nsf::method::setter for it.

XOTcl Next Scripting Language
Class C
C instforward f1 ...
C forward f2 ...

Object o
o forward f3 ...
# Define forwarder

Class create C { :forward f1 ... :class forward f2 ... } Object create o { :forward f3 ... }
# Define setter and getter methods in XOTcl.
#
# XOTcl provides methods for these.

Class C C instparametercmd p1 C parametercmd p2 Object o o parametercmd p3
# Define setter and getter methods in NX.
#
# NX does not provide own methods, but uses
# the low level framework commands, since
# application developer will only seldomly
# need it.

Class create C ::nsf::method::setter C p1 ::nsf::method::setter C -per-object p2 Object create o ::nsf::method::setter o p3

NX supports in contrary to XOTcl the method alias which can be used to register arbitrary Tcl commands or methods for an object or class under a provided method name. Aliases can be used to reuse a certain implementation in e.g. different object systems under potentially different names. In some respects aliases are similar to forwarders, but they do not involve forwarding overhead.

XOTcl Next Scripting Language
# Method "alias" not available
# Define method aliases
# (to scripted or non-scripted methods)

Class create C { :alias a1 ... :class alias a2 ... } Object create o { :alias a3 ... }

2.2.3. Method Modifiers and Method Protection

NX supports four method modifiers class, public, protected and private. All method modifiers can be written in front of every method defining command. The method modifier class is used to denote class-specific methods (see above). The concept of method protection is new in NX.

XOTcl Next Scripting Language
# Method modifiers
#
#   "class",
#   "public",
#   "protected", and
#   "private"
#
# are not available
# Method modifiers
#
#   "class",
#   "public",
#   "protected"
#
# are applicable for all kinds of method
# defining methods:
#
#    method, forward, alias, property
#
# The modifier "private" is available for
#
#    method, forward, alias
#
Class create C {
  :/method-definiton-method/ ...
  :public /method-definiton-method/ ...
  :protected /method-definiton-method/ ...
  :private /method-definiton-method/ ...
  :class /method-definiton-method/ ...
  :public class /method-definiton-method/ ...
  :protected class /method-definiton-method/ ...
  :private class /method-definiton-method/ ...
}

XOTcl does not provide method protection. In NX, all methods are defined per default as protected. This default can be changed by the application developer in various ways. The command ::nx::configure defaultMethodCallProtection true|false can be used to set the default call protection for scripted methods, forwarder and aliases, while ::nx::configure defaultPropertyCallProtection true|false can set the default protection for properties. The defaults can be overwritten also e.g. on a class level.

NX provides means for method hiding via the method modifier private. Hidden methods can be invoked only via the -local flag, which means: "call the specified method defined in the same class/object as the currently executing method".

XOTcl Next Scripting Language
# XOTcl provides no means for method hiding
# Hiding of methods via "private"
#
nx::Class create Base {
  :private method baz {a b} {expr {$a + $b}}
  :public method foo {a b} {: -local baz $a $b}
}

nx::Class create Sub -superclass Base {
  :public method bar {a b} {: -local baz $a $b}
  :private method baz {a b} {expr {$a * $b}}

  :create s1
}

s1 foo 3 4  ;# returns 7
s1 bar 3 4  ;# returns 12
s1 baz 3 4  ;# unable to dispatch method 'baz'

2.2.4. Method and Property Deletion

NX provides an explicit delete method for the deletion of methods and properties.

XOTcl Next Scripting Language
# XOTcl provides only method deletion with
# the equivalent of Tcl's "proc foo {} {}"
/obj/ proc foo {} {}
/cls/ instproc foo {} {}

# No support for property deletion
# Deletion of Methods
#
/obj/ delete method /name/
/cls/ ?class? delete method /name/

# Deletion of Properties /obj/ delete property /name/
/cls/ ?class? delete property /name/

2.3. Resolvers

The Next Scripting Framework defines Tcl resolvers for method and variable names to implement object specific behavior. Within the bodies of scripted methods these resolver treat variable and function names starting with a colon : specially. In short, a colon-prefixed variable name refers to an instance variable, and a colon-prefixed function name refers to a method. The sub-sections below provide detailed examples.

Note that the resolvers of the Next Scripting Framework can be used in the XOTcl 2.* environment as well.

2.3.1. Invoking Methods

In XOTcl, a method of the same object can be invoked via my, or in general via using the name of the object in front of the method name.

In NX, the own methods are called via the method name prefixed with a single colon. The invocation of the methods of other objects is the same in NX and XOTcl.

XOTcl Next Scripting Language
Class C
C instproc foo args {...}
C instproc bar args {
  my foo 1 2 3 ;# invoke own method
  o baz        ;# invoke other objects method
}
Object o
o proc baz {} {...}
Class create C {
  :method foo args {...}
  :method bar args {
     :foo 1 2 3 ;# invoke own method
     o baz      ;# invoke other objects method
  }
}
Object create o {
  :public method baz {} {...}
}

2.3.2. Accessing Own Instance Variables from Method Bodies

In general, the Next Scripting Language favors the access to an objects’s own instance variables over variable accesses of other objects. This means that in NX it is syntactically easier to access the own instance variables. On the contrary, in XOTcl, the variable access to own and other variables are fully symmetric.

In XOTcl, the following approaches are used to access instance variables:

  • Import instance variables via instvar and access variables via $varName

  • Set or get instance variables via my set varName ?value? or other variable accessing methods registered on xotcl::Object such as append, lappend, incr, etc.

  • Register same-named accessor functions and set/get values of instance variables via my varName ?value?

In NX, the favored approach to access instance variables is to use the name resolvers, although it is as well possible to import variables via nx::var import or to check for the existence of instance variables via nx::var exists.

The following examples summary the use cases for accessing the own and other instance variables.

XOTcl Next Scripting Language
Class C
C instproc foo args {
  # Method scoped variable a
  set a 1
  # Instance variable b
  my instvar b
  set b 2
  # Global variable/namespaced variable c
  set ::c 3
}
Class create C {
  :method foo args {...}
    # Method scoped variable a
    set a 1
    # Instance variable b
    set :b 2
    # Global variable/namespaced variable c
    set ::c 3
  }
}
... instproc ... {
   my set /varName/ ?value?
}
# Set own instance variable to a value via
# resolver (preferred and fastest way)

... method ... {
   set /:newVar/ ?value?
}
... instproc ... {
   my instvar /varName/
   set /varName/ ?value?
}
# Set own instance variable via
# variable import

... method ... {
   ::nx::var import [self] /varName/
   set /varName/ ?value?
}
... instproc ... {
   set /varName/ [my set /otherVar/]
}
# Read own instance variable

... method ... {
   set /varName/ [set /:otherVar/]
}
... method ... {
   set /newVar/ ${/:otherVar/}
}
... instproc ... {
   my exists /varName/
}
# Test existence of own instance variable

... method ... {
   info /:varName/
}
 ... method ... {
   ::nx::var exists [self] /varName/
}

2.3.3. Accessing Instance Variables of other Objects

XOTcl Next Scripting Language
/obj/ set /varName/ ?value?
# Set instance variable of object obj to a
# value via resolver
# (preferred way: define property on obj)

/obj/ eval [list set /:varName/ ?value?]
set /varName/ [/obj/ set /otherVar/]
# Read instance variable of object obj
# via resolver

set /varName/ [/obj/ eval {set /:otherVar/}]
... instproc ... {
   /obj/ instvar /varName/
   set /varName/ ?value?
}
# Read instance variable of object /obj/
# via import

... method ... {
   ::nx::var import /obj/ /varName/
   set /varName/ ?value?
}
/obj/ exists varName
# Test existence of instance variable of
# object obj

/obj/ eval {info exists /:varName/}
::nx::var exists /obj/ /varName/

2.4. Parameters

While XOTcl 1 had very limited forms of parameters, XOTcl 2 and NX provide a generalized and highly orthogonal parameter machinery handling various kinds of value constraints (also called value checkers). Parameters are used to specify,

  • how objects and classes are initialized (we call these parameter types Object Parameters), and

  • what values can be passed to methods (we call these Method Parameters).

Furthermore, parameters might be positional or non-positional, they might be optional or required, they might have a defined multiplicity, and value-types, they might be introspected, etc. The Next Scripting Framework provide a unified, C-implemented infrastructure to handle both, object and method parameters in the same way with a high degree of orthogonality.

Object Parameters were specified in XOTcl 1 primarily via the method parameter in a rather limited way, XOTcl 1 only supported non-positional parameters in front of positional ones, there were no value constraints for positional parameters, no distinction between optional and required, or multiplicity.

Furthermore, the Next Scripting Framework provides optionally Return Value Checking based on the same mechanism to check whether some methods return always the values as specified.

2.4.1. Instance Variables and Object Parameters

Object parameters are used for specifying, how objects are initialized (i.e. how instance variables are initialized, what parameters can be passed in for initialization, what default values are used, etc.). Object parameters are supported in XOTcl primarily via the method parameter, which is used in XOTcl to define multiple parameters via a list of parameter specifications. Since the term "parameter" is underspecified, NX uses a more differentiated terminology. NX distinguishes between instance variables with accessors (also called properties) and instance variables without accessors. To define a property, NX uses the method property, to define an instance variable without accessor, it uses the method variable. To define multiple properties in a short form (similar to XOTcl’s parameter), NX provides the method properties.

In a first step, we show the initialization of instance variables without accessors (using variable in NX), and then the definition of instance variables with accessors (using property).

XOTcl Next Scripting Language
# Define that instances of the class have
# a instance variables "x" and "y" initialized
# with some values

Class Foo Foo instproc init args { instvar x y set x 1 set y 2 } # Create instance of the class Foo Foo f1 # Object f1 has instance variables # x == 1 and y == 2
# The method "variable" is similar in syntax
# to Tcl's "variable" command. During object
# creation, the definition are used for the
# initialization of the object.

Class create Foo { :variable x 1 :variable y 2 } # Create instance of the class Foo Foo create f1 # Object f1 has instance variables # x == 1 and y == 2

While XOTcl follows a procedural way to initialize variables via the constructor init, NX follows a more declarative approach. Note, that the variable definitions are inherited from superclasses, which is straightforward in NX, while in XOTcl, the constructor has to call explicitly the constructor of its superclasses.

It is certainly as well possible to use constructors in NX in the same way as in XOTcl.

NX uses the same mechanism to define class variables or object variables.

XOTcl Next Scripting Language
# No syntactic support for creating
# class variables
# Define a class variable "V" with value 100 and
# an instance variable "x". "class variable" works
# similar to "class method".

Class create Foo { :class variable V 100 :variable x 1 }

In the next step, we define properties, i.e. variables with accessors.

XOTcl uses the method parameter is a shortcut for creating multiple properties. For every parameter definition, XOTcl creates as well a slot object, keeping an extensible set of meta-data for every parameter. Slot objects can be as well created in XOTcl directly via the method slots. NX provides a similar method named properties.

XOTcl Next Scripting Language
# Object parameter specified as a list (short form)
# "a" has no default, "b" has default "1"

Class Foo -parameter {a {b 1}} # Create instance of the class Foo Foo f1 -a 0 # Object f1 has instance variables # a == 0 and b == 1
# Object parameter specified as a list
# (short form); "a" has no default,
# "b" has default "1"

Class create Foo -properties {a {b 1}} # Create instance of the class Foo Foo create f1 -a 0 # Object f1 has instance variables # a == 0 and b == 1

Since every property defines a slot object, NX provides as well a scripted initialization for every slot object. Therefore, NX uses property to define a single property, similar in syntax to method parameters (a braced pair to denote a variable with a default). The method property can be used in NX on the class and on the object level (in XOTcl: just on the class level). When an property is created, NX does actually three things:

  1. Create a slot object, which can be specified in more detail using the init-block of the slot object

  2. Create an object parameter definition for the initialization of the object (usable via a non-positional parameter during object creation), and

  3. register an accessor function (setter), for wich the usual protection levels (public or protected) can be used.

The method variable in NX is similar to property, but it creates only slot objects in cases where needed, and it does not provide object parameters or accessors.

We show first the definition of properties simliar to the functionality provided as well by XOTcl and show afterwards how to use value constraints, optional parameters, etc. in NX.

XOTcl Next Scripting Language
# Object parameters specified via slots

Class Foo -slots { Attribute a Attribute b -default 1 } # Create instance of the class Foo and # provide a value for instance variable "a" Foo f1 -a 0 # Object f1 has a == 0 and b == 1
# Use the setter to alter instance variable "b" f1 b 100 # Use the accessor to output the value puts [f1 b]
# Object parameters specified via the method
# "property" (supports method modifiers and
# scripted configuration; see below)

Class create Foo { :property a :property {b 1} } # Create instance of the class Foo and # provide a value for instance variable "a" Foo create f1 -a 0 # Object f1 has a == 0 and b == 1
# Use the setter to alter instance variable "b" f1 b 100 # Use the accessor to output the value puts [f1 b]
# Parameters only available at class level
# Define a class property and an object
# property

Class create C { :property x :property {y 1} :class property cp } Object create o { :property op }
# Object parameter with configured slot,
# defining an attribute specific type
# checker

Class Person -slots { Attribute create sex -type "sex" { my proc type=sex {name value} { switch -glob $value { m* {return m} f* {return f} default { error "expected sex but got $value" } } } } }
# Object parameter with scripted
# definition (init-block), defining a
# property specific type checker

Class create Person { :property sex { :type "sex" :method type=sex {name value} { switch -glob $value { m* {return m} f* {return f} default { error "expected sex but got $value" } } } } }

XOTcl 1 did not support value constraints for object parameters (just for non-positional arguments).

NX supports value constraints (value-checkers) for object and method parameters in an orthogonal manner. NX provides a predefined set of value checkers, which can be extended by the application developer. In NX, the value checking is optional. This means that it is possible to develop e.g. which a large amount of value-checking and deploy the script with value checking turned off, if the script is highly performance sensitive.

XOTcl Next Scripting Language
# No value constraints for parameter
# available
# Predefined value constraints:
#    object, class, alnum, alpha, ascii, boolean,
#    control, digit, double, false, graph, integer,
#    lower, parameter, print, punct, space, true,
#    upper, wordchar, xdigit
#
# User defined value constraints are possible.
# All parameter value checkers can be turned on
# and off.
#
# Define a boolean property and an integer
# property with a default firstly via "properties",
# then with multiple "property" statements.

Class create Foo -properties { a:boolean {b:integer 1} }
Class create Foo {
   :property a:boolean
   :property {b:integer 1}
}

In XOTcl all object parameters were optional. Required parameters have to be passed to the constructor of the object.

NX allows to define optional and required object parameters (as well as method parameters). Therefore, object parameters can be used as the single mechanism to parameterize objects. It is in NX not necessary (and per default not possible) to pass arguments to the constructor.

XOTcl Next Scripting Language
# Required parameter not available
# Required parameter:
# Define a required property "a" and a
# required boolean property "b"

Class create Foo -properties { a:required b:boolean,required }
Class create Foo {
   :property a:required
   :property b:boolean,required
}

NX supports in contrary to XOTcl to define the multiplicity of values per parameter. In NX, one can specify that a parameter can accept the value "" (empty) in addition to e.g. an integer, or one can specify that the value is an empty or non-empty ist of values via the multiplicity. For every specified value, the value checkers are applied.

XOTcl Next Scripting Language
# Multiplicity for parameter not available
# Parameter with multiplicity
#   ints is a list of integers, with default
#   objs is a non-empty list of objects
#   obj is a single object, maybe empty

Class create Foo -properties { {ints:integer,0..n ""} objs:object,1..n obj:object,0..1 }
Class create Foo {
  :property {ints:integer,0..n ""}
  :property objs:object,1..n
  :property obj:object,0..1
}

The Object parameters provided by a class for the initialization of instances can be introspected via /cls/ info parameter (see [info_parameter]).

2.4.2. Method Parameters

Method parameters are used to specify the interface of a single method (what kind of values may be passed to a method, what default values are provided etc.). The method parameters specifications in XOTcl 1 were limited and allowed only value constraints for non positional arguments.

NX and XOTcl 2 provide value constraints for all kind of method parameters. While XOTcl 1 required non-positional arguments to be listed in front of positional arguments, this limitation is lifted in XOTcl 2.

XOTcl Next Scripting Language
# Define method foo with non-positional
# parameters (x, y and y) and positional
# parameter (a and b)

Class C C instproc foo {-x:integer -y:required -z a b} { # ... } C create c1 # invoke method foo c1 foo -x 1 -y a 2 3
# Define method foo with non-positional
# parameters (x, y and y) and positional
# parameter (a and b)

Class create C { :public method foo {-x:integer -y:required -z a b} { # ... } :create c1 } # invoke method foo c1 foo -x 1 -y a 2 3
# Only leading non-positional parameters
# are available; no optional positional
# parameters, no value constraints on
# positional parameters, no multiplicity, ...
# Define various forms of parameters
# not available in XOTcl 1

Class create C { # trailing (or interleaved) non-positional # parameters :public method m1 {a b -x:integer -y} { # ... } # positional parameters with value constraints :public method m2 {a:integer b:boolean} { #... } # optional positional parameter (trailing) :public method set {varName value:optional} { # .... } # parameter with multiplicity :public method m3 {-objs:object,1..n c:class,0..1} { # ... } # In general, the same list of value # constraints as for object parameter is # available (see above). # # User defined value constraints are # possible. All parameter value checkers # can be turned on and off. }

2.4.3. Return Value Checking

Return value checking is a functionality available in the Next Scripting Framework, that was not yet available in XOTcl 1. A return value checker assures that a method returns always a value satisfying some value constraints. Return value checkers can be defined on all forms of methods (scripted or C-implemented). Like for other value checkers, return value checkers can be turned on and off.

XOTcl Next Scripting Language
# No return value checking available
# Define method foo with non-positional
# parameters (x, y and y) and positional
# parameter (a and b)

Class create C { # Define method foo which returns an # integer value :method foo -returns integer {-x:integer} { # ... } # Define an alias for the Tcl command ::incr # and assure, it always returns an integer # value :alias incr -returns integer ::incr # Define a forwarder that has to return an # integer value :forward ++ -returns integer ::expr 1 + # Define a method that has to return a # non-empty list of objects :public class method instances {} \ -returns object,1..n { return [:info instances] } }

2.5. Interceptors

XOTcl and NX allow the definition of the same set of interceptors, namely class- and object-level mixins and class- and object-level filters. The primary difference in NX is the naming, since NX abandons the prefix "inst" from the method names.

Therefore, in NX, if a mixin is registered on the class-level, it is a per-class mixin, if the mixin is registered on the object level, it is a object-level mixin. In both cases, the method mixin is used. If a mixin is registered on the class object, one has to use the modifier class (in the same way as e.g. for defining methods).

2.5.1. Register Mixin Classes and Mixin Guards

XOTcl Next Scripting Language
/cls/ instmixin ...
/cls/ instmixinguard /mixin/ ?condition?
# Register per-class mixin and guard for
# a class

/cls/ mixin ... /cls/ mixin guard /mixin/ ?condition?
/cls/ mixin ...
/cls/ mixin guard /mixin/ ?condition?
# Register per-object mixin and guard for
# a class

/cls/ class mixin ... /cls/ class mixin guard /mixin/ ?condition?
/obj/ mixin ...
/obj/ mixinguard /mixin/ ?condition?
# Register per-object mixin and guard for
# an object

/obj/ mixin ... /obj/ mixin guard /mixin/ ?condition?

2.5.2. Register Filters and Filter Guards

XOTcl Next Scripting Language
/cls/ instfilter ...
/cls/ instfilterguard /filter/ ?condition?
# Register per-class filter and guard for
# a class

/cls/ filter ... /cls/ filter guard /filter/ ?condition?
/cls/ filter ...
/cls/ filterguard ...
# Register per-object filter and guard for
# a class

/cls/ class filter ... /cls/ class filter guard /filter/ ?condition?
/obj/ filter ...
/obj/ filterguard /filter/ ?condition?
# Register per-object filter and guard for
# an object

/obj/ filter ... /obj/ filter guard /filter/ ?condition?

2.6. Introspection

In general, introspection in NX became more orthogonal and less dependent on the type of the method. In XOTcl it was e.g. necessary that a developer had to know, whether a method is e.g. scripted or not and has to use accordingly different sub-methods of info.

In NX, one can use e.g. always info method with a subcommand and the framework tries to hide the differences as far as possible. So, one can for example obtain with info method parameter the parameters of scripted and C-implemented methods the same way, one one can get the definition of all methods via info method definition and one can get an manual-like interface description via info method parametersyntax. In addition, NX provides means to query the type of a method, and NX allows to filter by the type of the method.

2.6.1. List methods defined by classes

While XOTcl uses different names for obtaining different kinds of methods defined by a class, NX uses info methods in an orthogonal manner. NX allows as well to use the call protection to filter the returned methods.

XOTcl Next Scripting Language
/cls/ info instcommands ?pattern?
/cls/ info methods ?pattern?
/cls/ info instparametercmd ?pattern?
/cls/ info methods -methodtype setter ?pattern?
/cls/ info instprocs ?pattern?
/cls/ info methods -methodtype scripted ?pattern?
# n.a.
/cls/ info methods -methodtype alias ?pattern?
# n.a.
/cls/ info methods -methodtype forwarder ?pattern?
# n.a.
/cls/ info methods -methodtype object ?pattern?
# n.a.
/cls/ info methods -callprotection public|protected ...

2.6.2. List methods defined by objects

While XOTcl uses different names for obtaining different kinds of methods defined by an object, NX uses info methods in an orthogonal manner. NX allows as well to use the call protection to filter the returned methods.

XOTcl Next Scripting Language
/obj/ info commands ?pattern?
/obj/ info methods ?pattern?
/obj/ info parametercmd ?pattern?
/obj/ info methods -methodtype setter ?pattern?
/obj/ info procs ?pattern?
/obj/ info methods -methodtype scripted ?pattern?
# n.a.
/obj/ info methods -methodtype alias ?pattern?
# n.a.
/obj/ info methods -methodtype forwarder ?pattern?
# n.a.
/obj/ info methods -methodtype object ?pattern?
# n.a.
/obj/ info methods -callprotection public|protected ...

2.6.3. List class object specific methods

When class specific properties are queried, NX required to use the modifier class (like for the definition of the methods). In all other respects, this section is identical to the previous one.

XOTcl Next Scripting Language
/cls/ info commands ?pattern?
/cls/ class info methods ?pattern?
/cls/ info parametercmd ?pattern?
/cls/ class info methods -methodtype setter ?pattern?
/cls/ info procs ?pattern?
/cls/ class info methods -methodtype scripted ?pattern?
# n.a.
/cls/ class info methods -methodtype alias ?pattern?
# n.a.
/cls/ class info methods -methodtype forwarder ?pattern?
# n.a.
/cls/ class info methods -methodtype object ?pattern?
# n.a.
/cls/ class info methods \
   -callprotection public|protected ...

2.6.4. Check existence of a method

NX provides multiple ways of checking, whether a method exists; one can use info method exists to check, if a given method exists (return boolean), or one can use info methods ?pattern?, where pattern might be a single method name without wild-card characters. The method info methods ?pattern? returns a list of matching names, which might be empty. These different methods appear appropriate depending on the context.

XOTcl Next Scripting Language
/obj|cls/ info \
   [inst](commands|procs|parametercmd) \
   ?pattern?
/obj/ info method exists /methodName/
/obj/ info methods /methodName/
/obj|cls/ info \
   [inst](commands|procs|parametercmd) \
   ?pattern?
/cls/ ?class? info method exists /methodName/
/cls/ ?class? info methods /methodName/

2.6.5. List callable methods

In order to obtain for an object the set of artefacts defined in the class hierarchy, NX uses info lookup. One can either lookup methods (via info lookup methods) or slots (via info lookup slots). The plural term refers to a potential set of return values.

XOTcl Next Scripting Language
/obj/ info methods ?pattern?
/obj/ info lookup methods ... ?pattern?
# Returns list of method names
# n.a.
# List only application specific methods
/obj/ info lookup methods -source application ... ?pattern?
# Returns list of method names
# Options for 'info methods'
#
# -incontext
# -nomixins
# Options for 'info lookup methods'
#
# -source ...
# -callprotection ...
# -incontext
# -methodtype ...
# -nomixins
# n.a.
# List slot objects defined for obj
# -source might be all|application|baseclasses
# -type is the class of the slot object

/obj/ info lookup slots ?-type ...? ?-source ...? ?pattern?

# Returns list of slot objects

2.6.6. List object/class where a specified method is defined

info lookup can be used as well to determine, where exactly an artefact is located. One can obtain this way a method handle, where a method or filter is defined.

The concept of a method-handle is new in NX. The method-handle can be used to obtain more information about the method, such as e.g. the definition of the method.

XOTcl Next Scripting Language
/obj/ procsearch /methodName/
/obj/ info lookup method /methodName/
# Returns method-handle
/obj/ filtersearch /methodName/
/obj/ info lookup filter /methodName/
# Returns method-handle

2.6.7. List definition of scripted methods defined by classes

XOTcl contains a long list of info subcommands for different kinds of methods and for obtaining more detailed information about these methods.

In NX, this list of info subcommands is much shorter and more orthogonal. For example info method definition can be used to obtain with a single command the full definition of a scripted method, and furthermore, it works as well the same way to obtain e.g. the definition of a forwarder or an alias.

Another powerful introspection option in NX is info method parametersyntax which obtains a representation of the parameters of a method in the style of Tcl man pages (regardless of the kind of method).

XOTcl Next Scripting Language
# n.a.
/cls/ info method definition /methodName/
/cls/ info instbody /methodName/
/cls/ info method body /methodName/
/cls/ info instargs /methodName/
/cls/ info method args /methodName/
/cls/ info instnonposargs /methodName/
/cls/ info method parameter /methodName/
/cls/ info instdefault /methodName/
# not needed, part of "info method parameter"
/cls/ info instpre /methodName/
/cls/ info method precondition /methodName/
/cls/ info instpost /methodName/
/cls/ info method postcondition /methodName/
# n.a.
/cls/ info method parametersyntax /methodName/

2.6.8. List definition of scripted object specific methods

While XOTcl uses different names for info options for objects and classes (using the prefix "inst"), the names in NX are the same.

XOTcl Next Scripting Language
# n.a.
/obj/ info method definition /methodName/
/obj/ info body /methodName/
/obj/ info method body /methodName/
/obj/ info args /methodName/
/obj/ info method args /methodName/
/obj/ info nonposargs /methodName/
/obj/ info method parameter /methodName/
/obj/ info default /methodName/
# not needed, part of "info method parameter"
/obj/ info pre /methodName/
/obj/ info method precondition /methodName/
/obj/ info post /methodName/
/obj/ info method postcondition /methodName/
# n.a.
/obj/ info method parametersyntax /methodName/

For definition of class object specific methods, use the modifier class as shown in examples above.

2.6.9. List Slots and their definitions

XOTcl Next Scripting Language
# n.a.
# Return list of slots objects defined on the
# object or class
#
# -source might be all|application|baseclasses
# -type is the class of the slot object
# -closure includes slots of superclasses

/obj/ info slot objects ?-type ...? ?pattern?
/cls/ class info slot objects ?-type ...? ?pattern?
/cls/ info slot objects \ ?-type value? ?-closure? ?-source value? ?pattern?
# n.a.
# List definition of slots

/obj/ info slot definition \ ?-type value? ?pattern?
/cls/ class info slot definition \ ?-type value? ?pattern?
/cls/ info slot definition \ ?-type value? ?-closure? ?-source value? ?pattern?
/cls/ info parameter
# "info properties" is a short form of "info slot definiton"

/cls/ info properties \ ?-type value? ?-closure? ?-source value? ?pattern?
# n.a.
# List names of slots

/obj/ info slot names \ ?-type value? ?pattern?
/cls/ class info slot names \ ?-type value? ?pattern?
/cls/ info slot names \ ?-type value? ?-closure? ?-source value? ?pattern?
# n.a.
# List reachable slot objects defined for obj
# -source might be all|application|baseclasses
# -type is the class of the slot object

/obj/ info lookup slots \ ?-type ...? ?-source ... ?pattern? # Returns list of slot objects

2.6.10. List Object parameters

XOTcl Next Scripting Language
# n.a.
# Return parameter(s) provided by class for
# its instances; defines, how objects of this
# class can be configured. If name is provided
# only the named object parameter is returned
# otherwise the full list.
#
#
# Return object parameters with leading dashes
# for non-positional object parameters and
# defaults
/cls/ info parameter list ?name?

# Return just the names of the parameters /cls/ info parameter names ?name?

# Return the full parameter specs /cls/ info parameter definition ?name?

# Return the slot object(s) /cls/ info parameter slot ?name?

# Return in the Tcl parameter syntax /cls/ info parameter syntax ?name?

2.6.11. List Filter or Mixins

In NX all introspection options for filters are grouped under info filter and all introspection options for mixins are under info mixin. Therefore, NX follows here the approach of using hierarchical subcommands rather than using a flat namespace.

XOTcl Next Scripting Language
/obj/ info filter ?-guards? ?-order? ?pattern?
# ... info filter methods -order ... returns
# method-handles instead of triples
# (applies to all three variants)

/obj/ info filter methods \ ?-guards? ?-order? ?pattern?
/obj/ info filterguard /name/
/obj/ info filter guard /name/
/cls/ info filter ?-guards? ?-order? ?pattern?
/cls/ class info filter methods \
   ?-guards? ?-order? ?pattern?
/cls/ info filterguard /name/
/cls/ class info filter guard /name/
/cls/ info instfilter \
   ?-guards? ?-order? ?pattern?
/cls/ info filter methods \
   ?-guards? ?-order? ?pattern?
/cls/ info instfilterguard /name/
/cls/ info filter guard /name/
/obj/ info mixin ?-guards? ?-order? ?pattern?
/obj/ info mixin classes \
   ?-guards? ?-heritage? ?pattern?
/obj/ info mixinguard /name/
/obj/ info mixin guard /name/
/cls/ info mixin ?-guards? ?-order? ?pattern?
/cls/ class info mixin classes \
   ?-guards? ?-heritage? ?pattern?
/cls/ info mixinguard /name/
/cls/ class info mixin guard /name/
/cls/ info instmixin \
   ?-guards? ?-order? ?pattern?
/cls/ info mixin classes \
   ?-closure? ?-guards? ?-heritage? ?pattern?
/cls/ info instmixinguard /name/
/cls/ info mixin guard /name/

2.6.12. List definition of methods defined by aliases, setters or forwarders

As mentioned earlier, info method definition can be used on every kind of method.

XOTcl Next Scripting Language
# n.a.
/obj/ info method definition /methodName/
/cls/ ?class? info method definition /methodName/

2.6.13. List Method-Handles

NX supports method-handles to provide means to obtain further information about a method or to change maybe some properties of a method. When a method is created, the method creating method returns the method handle to the created method.

XOTcl Next Scripting Language
# n.a.
#
# List the method handle of the specified method,
# can be used e.g. for aliases. "handle" is the short
# form of "definitionhandle".
#
/obj/ info method handle /methodName/
/cls/ ?class? info method handle /methodName/
# # For ensemble methods (method name contains # spaces) one can query as well the registration # handle, which is the handle to the root of the # ensemble; the definiton handle points to the # leaf of the ensemble. # /obj/ info method registrationhandle /methodName/
/cls/ ?class? info method registrationhandle /methodName/
# # For aliases, one can query the original definition # via "info method origin" # /obj/ info method origin /methodName/
/cls/ ?class? info method origin /methodName/

2.6.14. List type of a method

The method info method type is new in NX to obtain the type of the specified method.

XOTcl Next Scripting Language
# n.a.
/obj/ info method type /methodName/
# n.a.
/cls/ ?class? info method type /methodName/

2.6.15. List the scope of mixin classes

NX provides a richer set of introspection options to obtain information, where mixins classes are mixed into.

XOTcl Next Scripting Language
/cls/ info mixinof ?-closure? ?pattern?
# List objects, where /cls/ is a
# per-object mixin

/cls/ info mixinof -scope object ?-closure? ?pattern?
/cls/ info instmixinof ?-closure? ?pattern?
# List classes, where /cls/ is a per-class mixin

/cls/ info mixinof -scope class ?-closure? ?pattern?
# n.a.
# List objects and classes, where /cls/ is
# either a per-object or a per-class mixin

/cls/ info mixinof -scope all ?-closure? ?pattern?
/cls/ info mixinof ?-closure? ?pattern?

2.6.16. Check properties of object and classes

Similar as noted before, NX uses rather a hierarchical approach of naming using multiple layers of subcommands).

XOTcl Next Scripting Language
/obj/ istype /sometype/
/obj/ info has type /sometype/
/obj/ ismixin /cls/
/obj/ info has mixin /cls/
/obj/ isclass ?/cls/?
/obj/ info is class
/obj/ ismetaclass /cls/
/obj/ info is metaclass
# n.a.
/obj/ info is baseclass
/obj/ object::exists /obj/
::nsf::object::exists /obj/

2.6.17. Call-stack Introspection

Call-stack introspection is very similar in NX and XOTcl. NX uses for subcommand the term current instead of self, since self has a strong connotation to the current object. The term proc is renamed by method.

XOTcl Next Scripting Language
self
self
current object
self class
current class
self proc
current method
self callingclass
current currentclass
self callingobject
current callingobject
self callingproc
current callingmethod
self calledclass
current calledclass
self calledproc
current calledmethod
self isnextcall
current isnextcall
self next
# Returns method-handle
current next
self filterreg
# Returns method-handle
current filterreg
self callinglevel
current callinglevel
self activelevel
current activelevel

2.7. Other Predefined Methods

XOTcl Next Scripting Language
/obj/ requireNamespace
/obj/ require namespace
# n.a.
/obj/ require method

2.8. Dispatch, Aliases, etc.

todo: to be done or omitted

2.9. Assertions

In contrary to XOTcl, NX provides no pre-registered methods for assertion handling. All assertion handling can e performed via the Next Scripting primitive nsf::method::assertion.

XOTcl Next Scripting Language
/obj/ check /checkoptions/
::nsf::method::assertion /obj/ check /checkptions/
/obj/ info check
::nsf::method::assertion /obj/ check
/obj/ invar /conditions/
::nsf::method::assertion /obj/ object-invar /conditions/
/obj/ info invar
::nsf::method::assertion /obj/ object-invar
/cls/ instinvar /conditions/
::nsf::method::assertion /cls/ class-invar /conditions/
/cls/ info instinvar
::nsf::method::assertion /cls/ class-invar
/cls/ invar /conditions/
::nsf::method::assertion /cls/ object-invar /conditions/
/cls/ info invar
::nsf::method::assertion /cls/ object-invar

2.10. Method Protection

As described above, NX supports method protection via the method modifiers protected and public. A protected method can be only called from an object of that class, while public methods can be called from every object. The method protection can be used to every kind of method, such as e.g. scripted methods, aliases, forwarders, or accessors. For invocations, the most specific definition (might be a mixin) is used for determining the protection.

3. Incompatibilities between XOTcl 1 and XOTcl 2

3.1. Resolvers

The resolvers (variable resolvers, function resolvers) of the Next Scripting Framework are used as well within XOTcl 2. When variable names or method names starting with a single colon are used in XOTcl 1 scripts, conflicts will arise with the resolver. These names must be replaced.

3.2. Parameters

The following changes for parameters could be regarded as bug-fixes.

3.2.1. Parameter usage without a value

In XOTcl 1, it was possible to call a parameter method during object creation via the dash-interface without a value (in the example below -x).

# XOTcl example

Class Foo -parameter {x y} Foo f1 -x -y 1

Such cases are most likely mistakes. All parameter configurations in XOTcl 2 require an argument.

3.2.2. Ignored Parameter definitions

In XOTcl 1, a more specific parameter definition without a default was ignored when a more general parameter definition with a default was present. In the example below, the object b1 contained in XOTcl 1 incorrectly the parameter x (set via default from Foo), while in XOTcl 2, the variable won’t be set.

# XOTcl example

Class Foo -parameter {{x 1}} Class Bar -superclass Foo -parameter x Bar b1

3.2.3. Changing classes and superclasses

NX does not define the methods class and superclass but allows to alter the class/superclass via configure. The class and superclass can be certainly queried in all variants with info class or info superclass.

# NX example

nx::Class create Foo Foo create f1 # now alter the class of object f1 f1 configure -class nx::Object

# alternate approach via Next Scripting Framework ::nsf::relation f1 class ::nx::Object

3.2.4. Info heritage

info heritage returns in XOTcl 1 the transitive superclass hierarchy, which is equivalent with info superclass -closure and therefore not necessary. In XOTcl 2 (and NX), info heritage includes as well the transitive per-class mixins.

3.3. Slots

All slot objects (also XOTcl slot objects) are now next-scripting objects of baseclass ::nx::Slot. The name of the experimental default-setter initcmd was changed to defaultcmd. Code directly working on the slots objects has to be adapted.

3.4. Obsolete Commands

Parameter-classes were rarely used and have been replaced by the more general object parameterization. Therefore, cl info parameterclass has been removed.

3.5. Stronger Checking

The Next Scripting Framework performs stronger checking than XOTcl 1 For example, the requiredness of slots in XOTcl 1 was just a comment, while XOTcl 2 enforces it.

3.6. Exit Handlers

The exit hander interface changed from a method of ::xotcl::Object into the Tcl command ::nsf::exithandler:

# NX example
::nsf::exithandler set|get|unset ?arg?